Tom,
Thanks for hunting down and posting the photos for me.
Crispin,
Thanks once more for the advice. I am weighing up whether to cut away
some more material from the bricks at either side of the throat to give
space to modify the grate angle and/or height once the stove is complete
or if I should modify the grate (quite possibly make a new one from bar
as you suggested) so that it can be lifted higher. I'm thinking it
would be best not to cut the bricks.
I was just looking at pricing for refractory board to make the sloping
sides in the upper part of the combustion chamber. I'm planning to have
three boards on all sides except where the door is.
I'm looking at 300mm x 200mm x 10mm - prices for soldering boards on
ebay, about £10 each). My father rang me and I told him what I'm up to
and he said he thought he had a sheet of fireproof roofing sheet
(asbestos replacement), which his brother had given him, in his shed
which should be big enough for what I need....
I've decided to add a second secondary air inlet on the front of the
burner, if for no other reason than to have it inputting some hot air up
by the window in the door to keep it clean. Also means I can play about
with the opening of primary air, flue damper, both secondary air inlets
and see what effect each has on the burn in the combustion chamber.
Best
Darren
On 25/11/2011 22:20, Crispin Pemberton-Pigott wrote:
Dear Darren
This is coming along well.
>Some photos are attached.
Got them, thanks.
>The door and hatch are made and have fire rope seals.
Good move. The hopper should have no air passing through it. There is
a pyrolysis zone above and near the primary air entrance, then a
burning zone further to the left (back of the grate) then a gas
burning zone above the rear of the grate ascending into the vertical
chimney.
>I've used a cast iron grate from an old coal stove that was the
perfect size.
It looks really. I think you should expect to make two changes once
you get going. One is to decrease the gap between the grate and the
ceramic bridge (we found 75mm is perfect for the fuel we are burning)
and to increase the 10 degree angle as a means for achieving this.
That means lifting up the front or near end of the grate. What would
make you want to try that would be excessive smoke caused by too much
fuel burning at once.
We ran quite a number of tests with different spacings like 65, 70, 75
and 85 and 90mm but all having 10 degrees of slope. The 75 mm straight
line distance was perfect. It gave an excess air ratio of 80-120% and
extraordinarily low PM values for much of the time. Because your fuel
is different you are going to have to do some hit and miss too until
you get a really clean flame about 15 minutes after ignition and
thereafter, no smoke. Hopefully it will perform as expected.
*At the moment all the secondary air is going in through some square
section pipe with an internal diameter of 20x20mm that enters at the
rear of the combustion chamber above the ceramic blocks. (shown as
blue square in stove2-7.jpg). Is this going to supply enough air or
should I put a similar sized secondary air inlet on the front of the
stove also?
This si going to be a very difficult question to answer without
equipment. Suppose you have enough air but your 100mm grate gap is too
big. It might incline you to provide more secondary air when you
really needed to reduce the burn rate. At some point, you have enough
power, so the air to match it is 'X' value. At a higher or lower burn
rate, you need a different amount. You are gonna tell us what you
need! I think the 20 x 20 may be too much but the fuel is different.
The easy test is to open and close it when there is only a little
smoke. It should make very little as it transitions from one firepower
to another as it lights up. Somehow the feedback from you hand closing
it will tell you which way to take things.
If you can't see any need for it, close it totally. It just add air
with no effect other than to cool the output gases. If you need it,
leave it open. We have not found any need to adjust the secondary
volume when it is running. Most of the secondary air enters through
the far end (back) of the grate where there is less fuel.
*Crispins GIZ design has an area where the combustion chamber tapers
wider (an expansion chamber?). I've drawn this in green in
stove2-7.jpg . How important is this?
The expansion is important but not critical. If you can, do it.
Professor Lodoysamba found that filling the combustion chamber with
wood and top-lighting it provided immediate heat for cooking, very low
emissions and easy ignition (automatic) of the fuel under the hopper.
Very clever.
When the fire reaches under the hopper, he had to shake the grate to
move fuel down onto the grate. That is all in terms of attention.
>What effect does it have on combustion? what would happen if I leave
them out?
The effect of the bricks/ceramic is to improve the combustion by
having hot walls. If you can't get real refractory bricks second hand,
use the hardest clay face bring you can find. It will last the longers
in terms of off the shelf material.
>...Hopefully I should finish and test fire it in the coming week.
Looking forward to more pictures and your comments.
>I've attached a diagram of how I'm now planning to build the grate and
throat area. (I took the liberty of modifying the GIZ diagram Crispin
kindly sent to explain an appropriate layout)
You construction matches the drawing.
As I'm only going to be burning wood and in my experience of wood
stoves all the wood burns to a light ash which would easily fall
through the grate I am planning to have a stationary grate. Am I
making the wrong decision here?
The grate gaps look large which will create extra charcoal that might
get snuffed in the ashes. Other than that it is fine. If the grate
turns out to be too 'coarse' you can drop in a set of 12mm steel
reinforcing bars welded into a grid. Works fine.
>Hoping to get it finished soon. Considering the number of people I've
been describing it to all year it would be good to also let them know
how it works!!! I've never seen anything like this in action - I'm
thoroughly intrigued.
You are going to have to try it with charcoal briquettes, wood
pellets, wood, chopped up wood, coal and switchgrass cubes if you want
us all to be happy! Good luck.
Regards
Crispin
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